It is generally recognized that the production of noxious oxides of nitrogen (NOx) which pollute the atmosphere are undesirable and in many cases are controlled by limits established by local, state and federal governmental regulations. The formation of NOx constituents in the exhaust gas products of an internal combustion engine must therefore be eliminated, minimized or at least maintained below some threshold limit or level.
It is generally understood that the presence of NOx In the exhaust of internal combustion engines is determined by combustion temperature and pressure as well as by the air/fuel ratio (lambda). An increase in combustion temperature causes an increase in the amount of NOx present in the engine exhaust. It is, therefore, desirable to control the combustion temperature in order to limit the amount of NOx present in the exhaust of an internal combustion engine.
One method suggested by the prior art for limiting or controlling the combustion temperature has been to recirculate a portion of the exhaust gas back to the engine air intake. It was reasoned in these early methods that since the exhaust gas is low in oxygen, this will result in a dilute combustion mixture which will burn at a lower temperature. The lower combustion temperature it was reasoned would, in turn, reduce the amounts of NOx produced during combustion.
Similarly, it had, until recently, been common practice to run an internal combustion engine at or near an ignition timing which produces peak combustion pressures which maximize combustion efficiency. However, unacceptably high levels of NOx may be produced in the combustion chambers when the engine operates at or near such conditions. In order to inhibit the formation and emission of NOx it is therefore desirable to limit the peak combustion pressure to a threshold value.
One technique suggested by the prior art for limiting combustion pressure involves the recirculation of exhaust gases through the induction passage of the combustion chamber since it is well-known that an increase in recirculation of exhaust gases will reduce peak combustion pressure and thus the attendant levels of undesirable NOx.
Therefore, it has become generally well-known that the formation of undesirable oxides of nitrogen may be reduced by recirculating a portion of the exhaust gas back to the engine air/fuel intake passage so as to dilute the incoming air/fuel mixture with inert H2O, and CO2. The molar specific heat of these gases and especially of CO2 absorbs substantial thermal energy so as to lower peak cycle temperatures and/or pressures to levels conducive to reducing NOx formation.
While NOx formation is known to decrease as the exhaust gas recirculation (EGR) flow increases to where it represents a threshold percentage of the exhaust gas constituents, it is also known that this is accompanied by a deterioration in engine performance including, but not limited to, an increase in engine roughness with increasing EGR. Therefore, one factor limiting the magnitude of EGR is the magnitude of EGR-induced performance deterioration or roughness that can be tolerated before vehicle drivability becomes unacceptable.
Early prior art attempts at solving these problems as well as other problems, such as complying with emission legislation which regulates other substances (e.g. smoke or particulate matter emissions) in addition to NOx, have employed various relatively simple mechanical schemes for directly controlling the position of an EGR control valve which may be operated by sensing a parameter such as throttle position, intake manifold pressure, exhaust back pressure, the air/fuel ratio, oxygen content, etc. Such early attempts to control EGR mechanically by sensing and shaping signals indicative of a parameter of engine performance or sensing engine flow as a function of venturi vacuum or exhaust back pressure, however, are not conducive to accuracy or programmability.
Several attempts have been made to obviate the problems of such simple mechanical control schemes. U.S. Pat. No. 4,174,027 to Nakazumi represents one such attempt. In accordance with this patent, an engine has a duct connecting gases in an exhaust gas crossover passage to an intake manifold. A flow control valve is provided for controlling the flow in the duct such that the valve is moved to an open position in response to control signals generated by a processor based upon input signals received from both a clutch-actuation detection device and a throttle valve-opening detection device. Once moved to the open position, the valve is kept in the open position for a predetermined time period as controlled by an electronic timer circuit.
U.S. Pat. No. 4,224,912 to Tanaka discloses an internal combustion engine which incorporates an exhaust gas recirculation system which has an exhaust gas recirculation flow control valve provided at a middle position of the exhaust gas recirculation passage, the control valve being operated in response to electronic signals generated by a processor depending upon a comparison between target and actual values of a vacuum pressure in a diaphragm chamber. An auxiliary valve is provided in the exhaust gas recirculation passage in series with the exhaust gas recirculation flow control valve so as to control the cross-sectional area of the exhaust gas recirculation passage in accordance with the opening amount of the throttle valve provided in the intake passage of the engine.
U.S. Pat. No. 4,142,493 to Schira et al. discloses a dosed loop EGR control system for an internal combustion engine having an intake system, an exhaust system, a throttle disposed within the intake system for controlling air flow therein, and a conduit coupling the exhaust system to the intake system for supplying exhaust gas back to the intake system. The EGR control system includes a first memory pre-programmed with a look-up table of optimal EGR values indicative of EGR valve position determined as a function of engine speed and throttle position and a second memory pre-programmed with the look-up table of optimal EGR values determined as a function of engine speed and absolute manifold pressure. The actual operating parameters of engine speed, throttle position and absolute manifold pressure are sensed and used by a processor to calculate control signals which are used to adjust the position of the EGR valve at a next scheduled valve adjustment time so as to regulate EGR flow as desired.
U.S. Pat. No. 5,611,204 to Radovanovic et al. discloses a gas flow network in combination with a highly turbocharged diesel engine for the blending of either EGR gas or blow-by gas from the crankcase vent with fresh charge air. In the diesel engine assembly which incorporates the flow network for EGR gas, a venturi conduit and control valve combination is positioned between tile intake manifold and aftercooler and is connected to a flow line carrying the EGR gas. When the turbocharged diesel engine assembly is configured with a flow path for blow-by gas, the venturi and control valve combination is positioned between the intake manifold and aftercooler and is connected to a flow line carrying blow-by gas. The control valve is controlled by an electronic system which generates control signals which utilizes only throttle position as an input (e.g. a sensed condition).
While the electronic systems disclosed in each of the above-mentioned patents may provide benefits over the rudimentary mechanical control schemes which were traditionally employed, they all suffer from disadvantages. One major disadvantage of all systems is that each relies upon various indirect parameters in order to calculate or estimate the flow of recirculating exhaust gas, and then to actuate various valves based upon such indirect parameters. More specifically, U.S. Pat. No. 4,174,027 to Nakazumi utilizes clutch-actuation detection and throttle valve-opening detection as input variables to the control system, U.S. Pat. No. 4,224,912 to Tanaka utilizes a vacuum pressure in a diaphragm chamber as a control variable, U.S. Pat. No. 4,142,493 to Schira et al. utilizes either engine speed and manifold absolute pressure or engine speed and throttle position as control variables, and U.S. Pat. No. 5,611,204 to Radovanovic et al. utilizes throttle position as a control variable.
In each of these patents, a processor utilizes the measured indirect parameters in conjunction with an algorithm and/or a look-up table in order to generate control signals for controlling the flow of recirculating exhaust gas. However, a problem exists in this approach in that by controlling the flow of recirculating exhaust gas based upon indirectly measured parameters, a source of error is introduced. For example, while the formula or look-up table may be accurate when a vehicle is first manufactured, after a period of use, deposits may form, or other conditions may exist which restrict the flow of recirculating gas, fresh air or both. As such, the algorithm or look-up table may no longer accurately reflect the flows within the system. For example, while the relationship between throttle position and recirculating gas flow may be known at the time of manufacture, after use deposits may form, the engine may become worm, etc., such that at a given throttle position, there may be significantly more or less recirculating gas flow. As such, indirectly controlling recirculating gas flow based upon throttle position may no longer be effective.
A related problem is that even when vehicles are first manufactured, there are always some at least small differences between individual vehicles. As such, in order for an EGR control system which employs measured indirect parameters in conjunction with an algorithm and/or a look-up table in order to generate control signals for controlling the flow of recirculating exhaust gas to be accurate, each individual system must be calibrated in order to calculate constants employed in the algorithm and/or values stored in the look-up table after the system is installed in the particular vehicle. This may be a time and cost intensive process.
All of these problems may be obviated by employing an EGR control system which employs directly sensed flow rate as a control variable, rather than control variables which are only indirectly related to flow rate.
What is desired, therefore, is a control system for controlling an exhaust gas recirculation system which is accurate and programmable, which provides for electronic control of the exhaust gas recirculation system, which remains accurate even after extended vehicle use, which does not require re-calibration after extended vehicle use, which does not generate control signals based solely upon sensed parameters indirectly related to flow rate, and which generates control signals based at least in part on sensed fluid flow.